Modulating system



Sept. 12, 1933. B LLEWELLYN 1,926,875

MODULATING SYSTEM -Fi1ed Jan. 7, 1932 SIGNAL SOURCE SOURCE CARRIE/soc/Rale- SIGML I SOURCE 23 CARR/ER 4 5 SOURCE ,3 I /6 24 7 D 2o 55%; melm w gl i l INVEN TOR E B. LLE WELL VN A T TORNE) Patented Sept. 12,1933 pars STAT I I l,92t,875

MODULA'IING SYSTEM Frederick B. Llewellyn, Montclair, N. 3., assignor toBail Telephone Laboratories, incorporated, New York, N. Y., acorporation of New York Application January 7, 1932. Serial No. 585,186

2 Claims.

This invention relates-to modulating systems of the space dischargetype, and more particularly to modulators in which the carrierwave isbalanced out or suppressed.

Heretofore balanced modulators have been employed extensively in systemsusing frequencies which are relatively low, for example, up to 50,000cycles per second but to a much less extent at higher frequencies. Inthe use of such modulators at the frequencies common in radiocommunication systems including both long wave and short wave systems,difficulty is encountered in meeting certain requirements easilysatisfied at the lower frequencies. One such requirement is that thecarrier wave be supplied through a circuit which can be adjustedsubstantially independently of the output circuit which transmits themodulated wave. It is essential that both the frequency and amplitude ofthe carrier wave supplied to the modulator be maintained very constantand this adjustment should not be disturbed by any adjustments orvariations which may take place in'theoutput circuit. At high pfrequencies, it is desirable that there should be a complete segregationof the respective circuits, which are provided for the carrier wave, themodulating current and the modulated output. It is further desirable inmost cases to conserve the energy of the modulating current bypreventing undue absorption of energy from the modulating current whenit is applied to the modulator.

In accordance with the present invention, a balanced modulator isprovided, comprising two electric discharge tubes, each of whichcontains in addition to the cathode, the anode and a control grid, afourth electrode constituting a screen grid which shields the anode fromthe control grid. The screen grid serves at the same time as a secondcontrol grid to which one of the input Waves may be applied. I

The invention is described in detail hereinafter in connection with theaccompanying drawing in which:

Figs. 1, 2 and 3 represent schematically several balanced modulatorcircuits embodying the invention.

Referring to Fig. 1, two electric discharge tubes 11 and 12 areconnected in a symmetrical or balanced arrangement. Each tube contains acathode 13, a control grid 14, an anode 15, and a screen grid 16 adaptedto electrostatically shield the anode 15 from the control grid 14. Thecathode 13 is assumed to be activated in any suitable manner. Thecontrol grid is negatively polarized While the screen grid and plate arepositively polarized. A carrier source 23 is connected to the primaryterminals A of a three-winding transformer 1'7, which is adapted tocouple the source to the two control grids 14, 14 in opposite phase. Asignal source 24 is connected'to the primary terminals C of a secondthree-winding transformer 19 adapted to couple the signal source to thescreen grids l6, 16 in opposite phase. A two-winding transformer 22 isarranged to respond to the sum of the plate currents of the two tubes,the summation current being made available at the pair of terminals 13.Due to the balanced arrangement of the circuits the carrier issuppressed at F, while modulation products or sidebands are delivered tothese terminals.

Fig. 2 shows a second balanced modulating system in which the carriersource 23 is connected at B to the input terminals of a two-windingtransformer 18 which is adapted for coupling to the control grids 1e, li in like phase. The signal source 24 is applied at C through thetransformer 19 to the screen grids 16, 16 in opposite phase. Themodulated output is available at E from a transformer 21 and due to thebalanced arrangement the unmodulated carrier is suppressed in the outputcircuit. The carrier source and the output circuit are substantiallyisolated from each other by the screening effects of the screen gridtubes. In this arrangement the screen grid is negatively polarized whilethe control grid is positively polarized.

Fig. 3 shows still another arrangement, in which the signal source isapplied at A to excite the control grids in opposite phase and thecarrier source is applied at D through a transformer 20, to the screengrids in like phase. As in the circuit of Fig. 2 the carrier issuppressed in the output circuit at The control grids are preferably.negatively polarized in order to prevent undue absorption. of the signalcurrents.

Analogous arrangements employing three-element discharge devices areshown in U. S. Patent 1,463,795, issued August 7, 1923 to J. R. Carson,but in the system of the present invention it is not necessary tosuperimpose the input and output circuits upon a single pair of tubeelements, there being available the additional circuit employing thepair of screen grids. Separate circuits are thus provided for thecarrier input, the signal input and the modulated output, respectively.The plate circuits are also substantially isolated from the control gridcircuits by the screen grids interposed between. Accordingly,

the circuits of terminals A and B are capable of adjustment withoutaifecting the circuits E and F, and vice versa. The degree ofindependence between the screen grid circuits and the plate circuits issomewhat less than between the control grids and the plates but isnevertheless fairly satisfactory. The screen grid circuits are alsoindependent of the control grid circuits to a large extent.

The arrangement of polarizing potentials shown in Fig. 2 is advantageouswhen the signal source is to be connected to the screen grids. In orderto prevent excessive loss due to absorption of signal currents in thescreen grid circuit the impedance of that circuit is increased bynegatively polarizing the screen grid. When this is done it is necessaryto polarize the control grid positively in order to neutralize theefiect of the space charge in the neighborhood of the cathode andprevent blockingof the tube.

What is claimed is:

1. A modulating system comprising a space discharge tube having acathode, an anode, a control grid and a screen grid, a source of carrierwaves and an output-circuit connected to the control grid and the anode,respectively, to secure substantial isolation of said source from saidoutput circuit by virtue of the screening chest of the screen grid, asource of modulating currents in circuit with the screen grid, saidscreen grid being negatively polarized to substantially preventabsorption of modulating currents in said tube, and a source of positivepolarizing potential connected to the control grid to ofiset theblocking effect of space charge in the neighborhood of the cathode.

2. A balanced modulating system of the suppressed carrier typecomprising a pair of electric discharge tubes, each including a cathode,an anode, a control grid and a screen grid, means for impressing anegative biasing potential upon the screen grids, a source of modulatingvoltage, means for connecting said source in opposite phase relation tothe respective screen grids, means for impressing a positive biasingpotential upon the control grids to counteract the effect of spacecharge in the neighborhood of the cathodes, a source of carrier waves,means for connecting said carrier Wave source in like phase relation tothe respective control grids, and a divided output circuit connected tothe anodes, said output circuit being substantially prevented fromreacting upon said control grids by the screening effect of said screengrids.

FREDERICK B. LLEWELLYN.

